mRNA decapping in Saccharomyces cerevisiae

Abstract

The major pathway of mRNA degradation in yeast occurs through deadenylation, decapping and subsequent 5' to 3' exonucleolytic decay of the transcript body. The products of the DCP1 and DCP2 genes are required for mRNA decapping. DCP1 encodes a conserved mRNA decapping enzyme. Dcp2p is a highly conserved protein that is required for the activation of Dcp1p. The Dcp2p contains a functional Muff motif that is required for its decapping function, suggesting that Dcp2p encodes a pyrophosphatase. These results suggest that Dcp2p hydrolyzes a specific pyrophosphate bond that either directly activates Dcp1p or removes a specific inhibitor of Dcp1p. In addition to Dcp2p, several additional proteins were identified that influence mRNA decapping. Edc1p and Edc2p are related proteins whose overexpression suppressed conditional mutations in dcp1 and dcp2, respectively. The Edc1 protein interacts in vivo with Dcp1p and Dcp2p. Based on similar genetic data for EDC1 and EDC2, the Edc2p also likely interacts directly with the mRNA decapping machinery. Edc1p and Edc2p may function to activate transitions in the decapping complex that lead to the Dcp2p-dependent activation of Dcp1p. The SBP1 protein was identified as an overexpression suppressor of a conditional dcp2 allele, termed dcp2-7. SBP1 overexpression also suppressed a conditional allele of the decapping enzyme (dcp1-2). In addition, the sbp1Delta was found to partially suppress the decapping defect of the dcp2-7 allele. This suggests that SBP1, which is a highly conserved RNA binding protein related to nucleolin, may influence the assembly or organization of the mRNP. Lastly, loss of function mutations in the previously uncharacterized IDC1 gene were shown to stimulate decapping in the presence of the dcp2-7 mutation. This suggests that the wild-type Idc1p inhibits mRNA decapping. Interestingly, the idc1 mutations described here represent the only known loss of function mRNA decapping suppressors that are not known to influence the rate of translation initiation, suggesting a more direct role for Idc1p in the inhibition of Dcp2p function. Combined, these results indicate that mRNA decapping is a highly controlled process involving the intricate and coordinated function of multiple proteins, in addition to the Dcp1p decapping enzyme.